ARSK and hyperinsulinemic hypoglycemia, familial, 4: The spectrum of MPSs has now expanded to include MPS X, caused by biallelic pathogenic variants in the ARSK (arylsulfatase K) gene, and a putative MPS IIIE subtype associated with ARSG (arylsulfatase G) deficiency, although the latter has so far been identified only in animal models and remains unconfirmed in humans [6,7].